Please let me know if you find out how to set a custom resolution in 'Windowed Fullscreen' specifically - this used to work in the Demo!

A custom resolution can be added via AMD / Nvidia panel and the game recognises the resolution but these resolutions only apply properly Windowed mode (trying different parameters in the GameUserSettings.ini file did not work for me).

Edit: Maybe it needed a reboot after setting the custom resolution because it is working the next day after posting!

Alternatively to AMD and Nvidia panel a custom resolution can be set using the ResolutionSizeX= / ResolutionSizeY= parameters from the GameUserSettings.ini file but both method failed for me other than in Windowed mode when trying to force a custom (ultrawide) resolution.

Thanks in advance!